Mitochondrial encephalomyopathies encompass a group of disorders that have impaired oxidative metabolism in skeletal muscles and central nervous system. Many compounds have been used in clinical trials on mitochondrial diseases, but the outcomes have been variable. It remains controversial whether treatment of mitochondrial diseases with coenzyme Q 10 is effective. This paper describes a case of mitochondrial myopathy, encephalopathy, lactic acidosis, strokelike episodes, and exercise intolerance successfully treated with coenzyme Q 10. Efficacy of this therapy in this patient is correlated to control of lactic acidosis and serum creatine kinase levels. Disappointingly, larger studies with coenzyme Q 10 failed to demonstrate a clear beneficial effect on the entire study population with regard to clinical improvement or several parameters of the oxidative metabolism. They suggest that the use of coenzyme Q in treatment of mitochondrial diseases should be confined to protocols. There is a confounding variation in phenotype and genotype, and the natural history of the disorders in individual patients is not accurately predictable. The unpredictable a priori efficacy of therapy suggests that a long-term trial of oral coenzyme Q may be warranted.